A Simple,Rapid,and Green Synthesis of Capped Gold Nanospheres and Nanorods Using Aqueous Extract of Azolla

Abstract

In the last few decades, an increasing commercial demand for metal nanoparticles is found due to their numerous applications in various fields such as electronics, catalysis in organic synthesis, material chemistry, energy, and medicine. Metallic nanoparticles are traditionally synthesized by wet chemical techniques, wherein the chemicals used are quite harmful and flammable. Herein, we reported a cheap and environment-friendly procedure for the synthesis of capped gold nanoparticles of different shapes from aqueous solution of tetrachloroauric acid (HAuCl₄) using aqueous extract of Azolla pinnata, blue-green algae used as a reducing as well as capping agent. The so-prepared gold nanoparticles were well characterized by UV-visible spectroscopy, transmission electron microscopy (TEM), and quasi-elastic light scattering (QELS) techniques. The TEM showed nearly uniform distribution of the particles in water, which is again confirmed by QELS. This is for the first time that aqueous extract of A. pinnata was used for the synthesis of gold nanoparticles.

Introduction

Nowadays, nanotechnology is opening new opportunities to chemists and biologist in the design and synthesis of assembled systems which can be used for biosensing, imaging, diagnosis and therapy. ^{1 –3} Controlling the size, shape, dispersity, and surface chemistry of nanoparticles (NPs) are some of the important properties that make this research field attractive, creating curiosity. The similar size of metal NPs and biomolecules is another advantage, when biological interactions have to be addressed. In last few decades, functionalized gold NPs (Au NPs) have attracted much attention towards research in the area of chemistry and biology, although their application in biomedicine is still in the initial stage. In the development of therapeutics, the multimerization of drugs on metal NPs may afford new tools that are more effective at intervening the biomolecular processes. Metal NP-based presentation of multiple ligands also creates a high concentration of drugs that can assist the targeted biological interaction/process. ^{4 –12}

Recently, Au NPs have also been the focus of intense research owing to their fascinating optical, electronic, and chemical properties and promising applications in biomedicine, sensing, and catalysis. They are highly useful for a wide range of processes including general nanotechnology and electronic manufacturing. An important and challenging task for Au NPs synthesis is the development of simple and versatile methods of preparation in a size- or shape-selected and eco-friendly manner. Conventional synthetic techniques based on the reduction of Au ions with sodium citrate or sodium borohydride, followed by surface modification of the produced particles with suitable capping ligands and organic solvents, raised environmental concerns because of the toxic compounds used in the process. These limitations invite new eco-friendly (green chemistry) methodology for the production of NPs with desired shape. The use of eco-friendly and nontoxic materials in the production of metal NPs is important for pharmaceutical and biomedical applications. Many researchers are now concentrating on biological entities for the synthesis of metal NPs. Scientific consensus emanating from several scientific investigations suggest that the roots, stems, fruits, and leaves of various herbs, spices, and plants contain high levels of powerful antioxidants such as photochemical constituents. ^{11,13 –21} These naturally occurring antioxidants have been proved to be nontoxic to living organisms and the environment. ^{1,22 –33} We report the utility of Azolla as reducing agents in the reduction of Au salts to corresponding Au NPs. Azolla (mosquito fern, duckweed fern, fairy moss, and water fern) is a genus of seven species of aquatic ferns, the only genus in the family Azollaceae. They are extremely reduced in form and specialized, looking nothing like conventional ferns but more resembling duckweed or some mosses. Azolla pinnata is a free-floating aquatic fern, which grows at a fast rate, doubling its biomass in 3–5 days and fixes atmospheric nitrogen by forming a symbiotic association with the blue-green algae, Azollae. In China and Vietnam, A. pinnata has long been used as both a green manure for rice and a fodder for poultry and livestock. A. pinnata has a symbiotic relationship with Azollae (nitrogen-fixing alga), and the A. pinnata symbiosis can fix 100–170 kgN/ha/years.

In the present work, we report the green synthesis of capped Au NPs using aqueous extract of A. pinnata, blue-green algae.

Experimental

Materials and Methods

Tetrachloroauric acid (HAuCl4) (purity > 99%) from Sigma-Aldrich Corporation (V.L. Enterprises, New Delhi, India) was used to prepare the aqueous solution of HAuCl₄. All glassware were rinsed with distilled water and dried in oven before use. Fresh algae, A. pinnata, were collected from the Department of Botany, University of Delhi, Delhi, India. UV-Visible spectra were recorded on Shimadzu UV-1800 having double beam in identical compartments for reference and solution fitted with 1-cm path length. Transmission electron microscopy (TEM) micrographs of the colloidal dispersions were obtained using a JEOL JEM-1400 (available in the USIC of Delhi University, Delhi, India) instrument operated at an accelerating voltage of 300 kV. Carbon-supported copper grids were used to support the colloidal dispersions. Specimens for imaging by TEM were prepared by evaporating a droplet of Au NP solutions onto carbon-coated copper grids. Quasi-elastic light scattering (QELS) of the colloidal dispersions were obtained.

Preparation of Capped Au NPS

Preparation of A. pinnata Extract (Solution A)

Algae, A. pinnata, were collected and dried in oven at 50 °C (3 days) and ground well. Then 5 g of the ground leaves were taken in 50 mL of double distilled water and stirred till reflux for 30 min in a 250-mL round bottom flask. Then, the reaction mixture was cooled at room temperature and the extract was filtered through Whatman 1 filter paper and the filtrate was collected and used.

Preparation of HAuCl₄ Solution (Solution B)

In a measuring flask (50 mL), 100 mg of HAuCl₄ was taken and dissolved in minimum quantity of double distilled water, which was made up to 50 ml with the solvent. The solution obtained was pale yellow (Figure 1A).

Figure 1.

Photographs of (A) Tetrachloroauric acid (HAuCl₄) solution, (B) Au NPs, (C) Thiazolidine-2,4-dione (TZD) capped Au NPs, and (D) TZD-collagen capped Au Nps. Au, gold; NPs, nanoparticles.

Synthesis of Au NPs with Aqueous Extract of Azolla

Next, 5 mL of solution B was taken in a vial (25 mL) and further diluted with 5 mL of water. Then aqueous extract of A. pinnata was added dropwise to the solution with constant stirring at 50 °C. The change in color of the solution was observed (yellow to pale green). It was further characterized using UV-visible, QELS, and TEM techniques (Figure 1B).

Synthesis of Thiazolidine-2,4-dione (TZD) Capped Au NPs with Aqueous Extract of A. pinnata

Solution B, 5 mL, was taken in a vial (25 mL) and further diluted with 5 mL of water. Two milliliters of aqueous solution of thiazolidine-2,4-dione were added to the solution and then the aqueous extract of A. pinnata was added to it dropwise with constant stirring at 50 °C. The color of the reaction mixture became ruby red. It was further characterized using UV-visible, QELS, and TEM techniques (Figure 1C).

Synthesis of TZD-Collagen Capped Au NPs with Aqueous Extract of A. pinnata

In a vial (25 mL), 5 mL of solution B was taken and further diluted with 5 mL of water. Then, 2 mL of aqueous solution of thiazolidine-2,4-dione and 4 mL of collagen solution (in water) were taken and then aqueous extract of A. pinnata was added to it dropwise with constant stirring 50 °C for appropriate time. The color of the reaction mixture became pink. It was further characterized using UV-visible, QELS, and TEM techniques (Figure 1D).

Results and Discussion

Here the aqueous leaf extract of A. pinnata was successfully used for the efficient and rapid synthesis of capped Au NPs. The phytochemicals present in the leaf extract are nontoxic and, at the same time, capable of stabilizing NPs as well as reducing in nature. In the present study, we synthesized protected Au nanorods and nanospheres using aqueous extract of A. pinnata in one step, without a reducing agent or a linker for stabilization. The reduction of HAuCl₄ was visually evident from the color change as in seen Figure 1 and was completed within 30 min. The characteristic surface plasmon resonance band of Au NPs was observed in the UV-visible spectrum, confirming the presence of capped Au NPs (nanorods) at 535 nm; while for TZD capped Au NPs and TZD-collagen capped Au NPs, it was observed at 541 and 549 nm, respectively (Figure 2). The TEM images showed rods for Au NPs and spherical for TZD capped Au NPs and TZD-collagen capped Au NPs, respectively (Figure 3). The QELS images showed that the particle size for rods is ∼200 nm, while for spherical it is 20 nm (Figure 4).

Figure 2.

UV-visible spectra of (A) Au NPs, (B) TZD capped Au NPs, and (Cc) TZD-collagen capped Au NPs. Au, gold; NPs, nanoparticles.

Figure 3.

Transmission electron microscopy images of (A) Au NPs, (B) TZD capped Au NPs, and (C) TZD-collagen capped Au NPs.

Figure 4.

Quasi-elastic light scattering images of (A) Au NPs, (B) TZD capped Au NPs, and (C) TZD-collagen capped Au NPs. Au, gold; NPs, nanoparticles.

Conclusion

In conclusion, the bioreduction of aqueous Au³⁺ ions by the aqueous extract of Azolla, the blue-green algae, has been demonstrated. The reduction of the metal ions using aqueous extract leads to the formation of Au NPs of different shapes. In the present study, we found that algae can also be considered the good source for the synthesis of capped Au nanorods and nanospheres. This green chemistry approach toward the synthesis of Au NPs of different shapes has many advantages, such as the ease with which the process can be scaled up, economic viability, and so on. Applications of such eco-friendly NPs in bactericidal, wound healing, and other medical and electronic applications makes this method potentially exciting for the large-scale synthesis of other inorganic materials (nanomaterials).